Electrochemical processes are used in a variety of applications from manufacturing and plating processes to electrochemical storage cells. Common to all of these processes is the conversion of chemical energy to electrical energy or electrical energy to chemical energy. During this conversion process, the materials involved change their physical properties.
All electrochemical cells share some common characteristics. A positive (+) electrode, or anode, is separated from the negative (+) electrode, or cathode, by a chemically inert separator that is capable of electrolytic conduction. In most battery systems the electrolyte is a liquid that consists of a salt or an acid that is dissolved in either water or an organic solvent. The electrolyte is usually a liquid which is added to the cell near the end of the manufacturing process. For some process systems the electrolyte may be a gel or a conductive solid (which may also act as the separator) which is more easily incorporated into the manufacturing process. The anode is where electrochemical reduction occurs during discharge. The cathode is where electrochemical oxidation occurs during discharge. Cathode and anode pairs can be combined in an electrochemical cell to increase the capacity and/or current capability of the system. Combinations of cells can be made to increase the voltage of a system. Such a combination is referred to as a battery.
Accurate knowledge of the state of charge of a cell or battery is often important for efficient and prolonged operation of the cell or battery. The increased use of secondary batteries as the main power source in many applications frequently makes knowledge of battery state of charge critical. Notwithstanding, the state of charge is a physical property that is not readily measurable. Consequently, several solutions have been proposed over the years for monitoring and measuring the state of charge in electrochemical storage batteries.
Most of these solutions predict the state of charge based upon measurements of battery voltage or current. However, the state of charge depends upon the extent to which an electrochemical reaction has occurred in a battery. Voltage or current measurements are an indirect and imprecise measure of the extent of the electrochemical reaction. Further, differences in the battery chemistry, even for the same battery type, manufacturing method, intended use, and use profile limit the usefulness of voltage and current measurements. Listed below is a brief explanation of the most widely used solutions for measuring state of charge.
The simplest and least satisfactory solution for measuring state of charge is simply to use the battery until the battery can no longer perform satisfactorily for its intended use. For rechargeable batteries, when this condition is met, the battery is charged for a predetermined period of time, and used again.
Another solution monitors the battery voltage. When the voltage falls below a predetermined value, a warning indicates the end of the discharge cycle so a system powered by the battery can be safely shut down. The battery is then charged for predetermined period of time and used again.
Other solutions provide a predetermined charge algorithm that is repeated each time a battery is charged. Most of these solutions, though, assume the battery is at a certain discharge state, regardless of the actual state of charge of the battery. More particularly, the charging algorithm does not distinguish between a battery that is fully discharged, and a battery that is only partly discharged.
In yet other solutions incorporating a charge algorithm, attempts are made to determine the battery state of charge by measuring battery temperature, voltage, and perhaps other properties. Based on this information, the battery state of charge is estimated by the charge algorithm.
Solutions involving flooded lead-acid batteries rely upon measuring the specific gravity of the battery electrolyte and temperature to determine the state of charge. This can be an accurate determination because it directly measures the extent of the electrochemical reaction in the battery, but is inconvenient and is best done in a static environment when the battery is not in use.
Solutions involving sealed lead-acid, alkaline (NiCd, NiFe, NiZn and NiMH), and lithium-ion systems rely on "across the terminal" measurements of battery voltage or battery impedance under various conditions to approximate the state of charge. Some of these solutions employ both measurements as a corroborative measure of the battery state of charge. Measurements across the terminals of a battery vary according to the chemistry of the battery. Even identical battery chemistries have different measurement parameters for different battery configurations. For example, with nickel-cadmium batteries the behavior of sealed and vented cells differs because of the limited amount of electrolyte in the sealed cells. There are also differences that depend on whether the cell is designed for high capacity (dense electrodes) or high current capability (higher porosity electrodes). Even when cells are supposed to be identical, the statistical differences in manufacture will limit the effectiveness of across the terminal measurements.
In other solutions, battery current is monitored to estimate the amount of charge applied or drawn from a battery. These types of solutions are limited in that they do not account for several factors, such as: the limited efficiency inherent in battery reactions; parasitic reactions involving gas generation during charge; or parasitic reactions involving discharge of the electrode substrates during over discharge. These factors not only affect the initial accuracy of this approach, but also may vary substantially over time and battery usage. Therefore, these solutions require correction algorithms to account for the inaccuracies caused by variations in the noted factors. In lab environments these solutions can accurately determine the state of charge of a battery. However, in actual applications, the correction algorithms are virtually impossible to predict.
One proposed solution to the problem of measuring battery state of charge is disclosed in U.S. Pat. Nos. 5,132,626 and 5,250,903 to Limuti et at. These patents disclose a coil encircling a battery or cell, with the central axis of the coil being in a plane that is parallel to the planes of the battery plates (anodes and cathodes). The coil induces voltage on the battery plates that cause eddy currents to circulate in a manner similar to the flow of currents in a standard laminated `C` or `E` core transformer. According to the patents, the state of charge in flooded lead acid storage batteries is measured by sensing the available "metallic volume" of the battery. The coil, however, is not protected with shielding from electromagnetic interference ("EMI"), nor is there any attempt to prevent the coil from detecting any surrounding conductive or permeable materials. Further, the magnetic flux generated by the coil does not impinge on the plates in an optimal direction. In addition, there is no possibility of isolating the response of one plate of the battery from the response of another plate.
The susceptibility of the coil to EMI interference means that the solution for measuring state of charge proposed in these patents is not capable of consistent performance in most practical applications. An attempt was made to use the solution disclosed in these patents in a laboratory to determine the state of charge of a battery. Extremely poor signal conditions were observed. Measurements consisted mostly of noise, with a small, almost unusable, signal response. The poor laboratory results suggests that it would be impractical to use this solution in typical applications in which electrical noise, thermal, or mechanical variances would contribute significant measurement errors.
Another solution is proposed in U.S. Pat. No. 5,093,624 to Stevenson. This patent discloses placing a coil adjacent the side of a lead-acid battery that is nearest the negative plate of the battery. An alternating voltage is then applied to the coil, which the patent asserts causes an eddy current to circulate on the negative plate of the battery. According to the patent, the alternating current in the coil increases as the battery charges, to a maximum, and then begins to decrease. Thus, the alternating current in the coil is monitored to determine the state of charge of the battery.
However, there is no attempt to protect the coil from EMI, or to prevent the coil from detecting surrounding conductive or permeable materials. Hence, this is also an impractical solution.
The problems associated with electronic interference from external sources (EMI), electronic radiation from the sensing coil causing interference and compatibility problems with other equipment (EMC), sensor temperature stability, adjacent material sensing interference, signal-to-noise ratio optimization, as well as sensing parameter optimization, appear to all have been ignored in existing patents. Accordingly, the present invention provides an improved solution to the problem of measuring the state of charge of a battery or other electrochemical system.